For screwing or unscrewing threaded connection elements, and more particularly large connection elements such as bolts, automatic means such as screwing and unscrewing robots are generally used.
The unscrewing operation is usually carried out by fastening the rod of a traction jack onto the free part of the connection element and then exerting, via the jack, a tensile force on the connection element so as to cause the said element to elongate, which subsequently pulls the tightening nut with it so as to move it away from the bearing face. The connection element is then driven in rotation in the opposite direction of the thread line by a screwing and unscrewing robot.
The first stage of the unscrewing, which consists in unlocking the connection element, is often the most awkward, and it is at the beginning of this stage that certain difficulties appear.
These difficulties may result from the threads becoming clogged up, and from a mechanical stiff point.
In the first case, it is evident that, in order to act effectively, an unscrewing torque must be exerted of a value greater than that initially provided.
This state of affairs entails, in the region of the connection between the traction jack and the connection element, additional and therefore parasitic forces, the effect of which is to further the slipping, either intermittent or permanent, of the said connection.
In the second case, the situation is particularly awkward. The occurrence of the mechanical stiff point may be caused either by the presence of an isolated stiff body lodged between the threads or by the creep caused by an excessive tightening force or, alternatively, a force during operation.
Whatever the type of anomaly to be eliminated, the means employed up until now do not enable the problem to be effectively overcome and generally require a fairly long service time, which can be prejudicial to the safety of the personnel charged with these services, since they may be performed in an area which has often been rendered dangerous by it being subjected to high doses of radiation or to chemical pollution.